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The age at which a woman had her first period can help predict her children's risk of obesity, say UK researchers.

A study of 6,000 children found those born to mothers with an early puberty were more likely to grow rapidly as babies and be overweight as children.

This faster growth pattern is also linked to obesity in adulthood.

The findings could help identify children at risk of weight problems early on, the Public Library of Science Medicine (PLOS) report concludes.

It is already known that age at which a girl has her first period - or reaches "menarche" - is largely inherited.

And women who start their periods early are at increased risk of obesity in later life, and are likely to be overweight even before puberty.

In the latest study, mothers who began their periods before age 11 were five times more likely to be obese than mothers who had their first period after the age of 15.

Children of mothers who had early first periods were taller by the age of nine and weighed more. Girls were also more likely to start their periods before the age of 11.

Those whose mothers had their first period under the age of 11 were three times more likely to be obese than those who started their periods after the age of 15.

The researchers also looked in more detail at growth measurements from birth to nine years in 900 children, and found that mothers' age at first period was associated with faster growth in weight and height in children up until the age of two years.

Children who have a fast growth pattern tend to start puberty earlier, but stop growing sooner - so they may not be particularly tall as adults.

Lifelong pattern

Lead researcher Dr Ken Ong, paediatric endocrinologist at the Medical Research Council Epidemiology Unit at Cambridge University, said it was now clear that this growth pattern starts at birth.

"Beyond links to early puberty, most importantly this growth pattern appears to lead to an increased risk of obesity that lasts from childhood through to adult life."

He added there could be a genetic link - perhaps related to appetite - or that the findings may be explained by feeding behaviours or patterns that run in families.

"Knowing that rapid infancy weight gain, early puberty and obesity run together in families may help us identify which children to best target our efforts at right from birth."

He added that parents should be encouraged to breastfeed rather than bottle-feed.

The team are also beginning a study to look at trying to avoid excessive calorie consumption in babies who are formula fed.

Dr Tabitha Randell, consultant paediatric endocrinologist at University Hospital Nottingham, said the big point was that heavier mothers had heavier children.

"Age at menarche is strongly related to your weight and it's well recognised that it's getting earlier and earlier."

But she said that environmental factors were just as important as genetics.

"It would be very interesting to have a look at the effect of breastfeeding.

"It sets up feeding patterns. You lay down your number of fat cells in the first couple of years of life," she said.

Scientists say they have successfully made immature sperm cells from human bone marrow samples.

If these can be grown into fully developed sperm, which the researchers hope to do within five years, they may be useful in fertility treatments.

But experts have warned the findings from the German study should be interpreted with caution at this very early stage.

And proposed new laws would ban their use in fertility treatments in the UK.

How sperm is made from bone marrow tissue?

The government's recent fertility White Paper proposes a ban on using artificially created sperm or eggs in assisted reproduction.

The researchers from the Universities of Göttingen and Münster and the Medical School of Hannover isolated adult stem cells - cells that have the ability to become many types of tissue in the body - from bone marrow samples taken from male volunteers.

Spermatagonial cells

Normally these stem cells from the bone marrow would develop into the different cell types in muscle tissue.

But the researchers induced a small number of them to develop into what appeared to be spermatagonial cells - cells found in the testes which would normally develop into mature sperm cells.

This is the first time human spermatagonial cells have been made artificially in this way.

And lead researcher Professor Karim Nayernia, now at the North-east England Stem Cell Institute based at the Centre for Life in Newcastle upon Tyne, said he hopes his investigations will mean he might one day be able to treat young men rendered infertile by chemotherapy.

He said: "We're very excited about this discovery.

"Our next goal is to see if we can get the spermatagonial cells to progress to mature sperm in the laboratory and this should take around three to five years of experiments."

He acknowledged that the law may be a stumbling block.

Professor Harry Moore of the Centre for Stem Cell Biology at the University of Sheffield said: "This finding is of interest but we really need to be very cautious about the interpretation."

The changing of stem cell types observed by the researchers, from pre-muscle to pre-reproductive cells, is known as trans-differentiation.

Caution needed

And Professor Moore said nearly all similar observations of trans-differentiation in adult stem cells had not been confirmed when tested rigorously, as the results could be misleading.

He said: "This is a fast moving field but we are still many years away from developing any therapies for infertility using such techniques."

And he warned that manipulating stem cells to develop into mature sperm could cause permanent genetic changes in the sperm, making the cells unsafe to use in fertility treatments.

However, Dr Allan Pacey, secretary of the British Fertility Society, said: "This kind of work will be very useful in helping us understand the biology of how sperm are made.

"That in itself may provide some insight into why things sometimes go wrong and may lead to new approaches to treating male infertility."

The findings were presented at an international fertility conference and are published in Gamete Biology: Emerging Frontiers on Fertility and Contraceptive Development.

1) Stem cells isolated from bone marrow of male volunteers

2) In laboratory, stem cells cultured and identified. Some induced to become spermatological cells - which normally become sperm cells

3) Transplantation of cells into testicle - so far performed in mice